(a) Field of the Invention
This invention relates to an optical equipment of a laser Doppler velocimeter which utilizes an interference of two laser beams to measure velocity, for example flow in a pipe, combusting flow, flow in a engine, and so on. Especially, this invention relates a optical equipment of a fiber laser Doppler velocimeter probe and an optical equipment of a manipulator to transmit a laser into a fiber.
(b) Description of the Prior Art
Generally, in laser Doppler velocimeters, a laser beam is split into two beams and collimated. These two beams are focused by a front lens and form a measurement volume wherein an interference fringe are made. A particle in flow, for example a dust, traveling the measurement volume causes scattering light, which is detected at a photomultiplier and transformed into electric Doppler burst signals. The velocity of the flow is obtained by processing the Doppler signal by a signal processing system.
In fiber LDVs, optical fibers are used to transmit laser power to a measurement volume and to transmit a scattered light to a photomultiplier.
As fiber LDV probes, a compact type of the fiber LDV probe as shown in FIG. 6 comprises a Selfoc microlens to collimate the laser beam, front lens, c, to focus two or four beams and make measurement volume, o, receiving lens to collect scattering light from the measurement volume and focus on the end of a receiving fiber, e, and a receiving fiber to transmit the scattered light to a photomultiplier. While, a large type fiber LDV as shown in FIG. 7 comprises a Selfoc micro lens, b, b, to collimate the laser beams out of the fibers, beam expander lens, g, h, to expand a beam diameter to increase the scattered light intensity, front lens, receiving lens, and a receiving fiber.
In a compact type of fiber LDV, a beam out of a Selfoc micro lens can not be expanded due to its compact size so that the measurement volume becomes large which decrease a signal-to-noise ratio. Furthermore, the troublesome light scattered when laser beam passes through a lens causes noise and decrease an accuracy of the measurement. A space filter to cut off the troublesome light scattered can not be used in the compact fiber LDV due to its size.
In a large type of fiber LDV probe, a beam expander is used to make the beam diameter large to make the measurement volume small to increase a light scattered intensity and a complicated adjustment of an optics is required.
At a manipulator, a focused laser beam has to be positioned in a core of a fiber, a, as shown in FIG. 5, so that three dimensional traversings, x, y, z, and two angle adjustments, 0, are required. Since the core diameter of the fiber is 3 to 4 um, a resolution of the adjustment of less than one micron is needed which can hardly be achieved even using a micro meter. Especially, a polarization preserving single mode fiber has small numerical aperture, NA. This means that when a laser beam focused to transmit laser power into a fiber the focused diameter has to be smaller than the core diameter of the fiber and NA of the focusing laser beam has to be smaller than the fiber's NA.